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NRF9160 GPS Speed Accuracy

Hi,

How do we get the GPS speed accuracy out of the module? We have used other GPS modules in the past which have output the speed accuracy which we use to qualify the data.

I have noticed occurrences where the GPS fix looks fine, but only when validating the data myself is it obvious the speed is way out. For example, when leaving a tunnel I got a GPS fix with a PDOP of 2.2, >= 4 satellites and a position accuracy 14m, but with a speed of 169km/h which is double what I was doing (speed limit on the road is 80km/h). Here is a snapshot of the data from our platform:

Kind regards,

Thomas

Parents Reply
  • That is truly a shame. 

    We are a tracking device manufacturer and we use the GPS speed in our logic to detect movement and our customers use the speed to monitor drivers in their fleets and we wouldn't want to get a driver fired because the GPS receiver was declaring their were doing double the speed limit when they weren't.

    We use the speed accuracy in our current designs to discard GPS fixes that are not accurate enough.

    How many fixes does your team recommend discarding before accepting a GPS speed?

Children
  • First, are you using continous mode?

    If so, there is a linear filter internally in the GPS, which will help improve the accuracy over multiple fixes.

    However, you can still for various reasons (e.g. multipaths, bad satellite visibility/geometry, etc.) get occasional outliers (for both position and speed). This is probably what happened in your case.

    Linear filters are generally not very good at filtering out these outliers. In this case, where there was a spike of 160km/h when the actual speed was 80km/h, it would take an average of 10 samples to get the error to 10%.

    One way to protect yourself against these outliers is to use a non-linear filter, e.g. use the median of those 10 samples instead of the average. Another possibility is to drop samples that defy the laws of physics. E.g. A car takes 10 seconds to go from 0 to 100 km/h, also 10 seconds to go from 80 to 120 km/h (assuming the kind of cars I am driving), so omit any samples that do not fit in these limits etc.

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